I F S
ISO- Range- ISO 50-65000 (Best Range 100-1600 in Full Frame)
Controls Amount of Light and Noise (Grain) coming into your camera.
Higher the ISO Higher the Light and Higher the Grain
Used in Low Light Conditions and when you want to use a Faster Shutter Speed
F - Depends on Lens- F1.2-F22, each lens has own sweet spot
Aperture controls the amount of Image that is in Focus.
Higher Aperture Value means closed Aperture and Lower Aperture Value means open Aperture.
Higher Value (F5.6, F8, F11) means more Depth of Field and more Planes in Focus
Lower Aperture Value (F1.8, F2.8) means lesser Planes in Focus, more Bokeh, but can result in Out of Focus Images if subject is moving
S - Range- Bulb to 1/8000secs (Best Shutter speed is above 1.5x Lens Focal Length)
Controls how much time the Shutter is Open. Quicker the Shutter Speed, sharper are the pictures in terms of movement.
Ideally must shoot at as high a shutter speed as light permits.
You may need to amp up the ISO and open up the Aperture to compensate for the loss of Light
Now a days most cameras have very good sensors and hence do not be afraid to shoot at ISO other than your native (ISO 100). I usually shoot between 400 to 1600 even for my Ports. Even at higher ISO there is not much visible grain, and if there is you can accept that as a film look, or easily de-noise it in post process. But fixing other issues that arise due to not using correct ISO for eg a blurry shot or clipped shadows are more difficult to recover in post process and also cannot usually be used as aesthetic. Always shoot a technically sound image pushing your camera to the maximum and also keeping in mind your post process capabilities. Instead you can concentrate better on framing and composing your shots. Tripods are boring ;)